The SharePoint Team posted a great blog article on architecting for SharePoint deployments. There’s a lot of great content in that article, and it really lays out what I think is a critical tenet: Content Is King. Figure out how long your content will need to be around, and make many of your architectural decisions around that.
The article has some great discussion on laying out SharePoint installations: How you should host on server farms for particular implementations, how quotas should be addressed for those implementations, what considerations to give for self-service site creation, how to deal with content expiration for that, etc. It really is great reading.
(BTW, if you’re in the SharePoint space make sure you’re subscribed to Steve Pietrek’s SharePoint Link blog. Steve gets a great amount of highly useful links out on a daily basis.)